A Michelson two-bean interferometer used for a FTIR has a configuration wherein infrared light emitted from an infrared light source is divided by a beam splitter 70 in two directions, toward a stationary mirror and a movable mirror, whereby the infrared light reflected back from the stationary mirror and the infrared light reflected back from the movable mirror are combined in the beam splitter and sent along a single optical path. Here, as the movable mirror is moved back and forth in the direction of the input light axis, the difference in optical path length of the two split light beams changes, and the combined light becomes an interference light signal (interferogram) whereof the light intensity changes according to the position of the movable mirror.
FIG. 6 is a drawing which illustrates the configuration of the main parts of a conventional FTIR. FTIR 201 comprises a main interferometer principal part 40, an infrared light source unit 10 which emits infrared light, an infrared light detection unit 20 in which sample S is placed, a movable mirror velocity information detection unit 30, and a computer 250 (for example, see Patent Literature 1).
Infrared light source unit 10 comprises an infrared light source which emits infrared light, a converging mirror, and a collimator mirror. By means of this, the infrared light which is emitted from the infrared light source is outputted via the converging mirror and collimator mirror to beam splitter 70 of main interferometer principal part 40.
Infrared light detection unit 20 comprises a parabolic mirror, an ellipsoidal mirror, an infrared detector 21 which detects an interferogram (IFG signal), and a sample placement unit in which a sample S is placed. By means of this, the light converged by the parabolic mirror is shined on the sample S, and light which passes through (or is reflected by) the sample S is converged by the ellipsoidal mirror toward the infrared detector 21.
The main interferometer principal part 40 comprises a case 42 with an inside space, a movable mirror unit 260 is arranged in the top part of FIG. 6, a beam splitter 70 is arranged in the middle part of FIG. 6, and a stationary mirror unit 80 comprising a stationary mirror 85 is arranged in the lower part of FIG. 6.
FIG. 7 is a vertical cross-sectional view of mobile mirror unit 260. The mobile mirror unit 260 comprises a ceiling plate 264, a bottom plate 265 and two plates 266, 267. The top end part of plate 266 is coupled to the left side part of the bottom surface of ceiling plate 264, and the bottom end part of plate 266 is coupled to the left side part of the top surface of bottom plate 265. Furthermore, the top end part of plate 267 is coupled to the right side part of the bottom surface of ceiling plate 264 and the bottom end part of plate 267 is coupled to the right side part of the top surface of bottom plate 265.
As a result, the bottom plate 265 is suspended by means of plates 266 and 267 so as to be movable in the left-right direction with respect to the ceiling plate 264.
A yoke 268 is secured to the middle part of the bottom surface of the ceiling plate 264, and a magnet 269a and pole piece 269b are secured to the yoke 268 by a bolt 270.
A voice coil 272 is secured via angle plate 271 to the central part of the top surface of the bottom plate 265. A lead wire 273 is electrically connected to the voice coil 272, and the voice coil 272 is designed to move through the magnetic field formed by magnet 269a, yoke 268 and pole piece 269b. 
A mirror holder 261 is secured to the left side of the top surface of the bottom plate 265, and the central part of disc-shaped movable mirror 262 is secured to the top end part of mirror holder 261. As a result, when electric current is made to flow via lead wire 273 to voice coil 272, the voice coil 272 receives magnetic force due to the magnetic field formed between yoke 268 and pole piece 269b, and when the bottom plate 265 moves in the left-ride direction, the movable mirror 262 also moves in the left-right direction M.
Furthermore, the ceiling plate 264 of the movable mirror unit 260 is attached to the case 42 using a screw and washer 263.
With a main interferometer principal part 40 of this sort, the infrared light emitted from infrared light source unit 10 is shined onto beam splitter 70, and is split by the beam splitter 70 in two directions, toward the stationary mirror 85 and movable mirror 262. The infrared light reflected back from the stationary mirror 85 and the infrared light reflected back from the movable mirror 262 return to the beam splitter 70, and are combined by the beam splitter 70 and sent along an optical path toward infrared light detection unit 20. Here, the movable mirror 262 moves back and forth in reciprocating fashion in the input light axis direction M, so the difference in optical path length of the two split beams changes periodically, and the light which heads from the beam splitter 70 to the infrared light detection unit 20 becomes an interferogram whereof the amplitude varies over time. Furthermore, the interferogram which has passed through sample S is converged toward infrared detector 21. FIG. 8 is a drawing which shows an IFG signal illustrating an example of the relationship between light intensity and movable mirror position.
Furthermore, a movable mirror velocity information detection unit 30 which detects movable mirror velocity information is provided in FTIR 201. The movable mirror velocity information detection unit 30 performs velocity information detection using laser light, and comprises a He—Ne laser light source unit 31 which emits laser light, half-mirrors 32 and 33 which reflect laser light, and laser light detection unit 34 which detects laser light information (for example, see Patent Literature 2).
With this sort of movable mirror velocity information detection unit 30, the laser light emitted from the He—Ne laser light source unit 31 is shined onto beam splitter 70, and is divided by the beam splitter 70 in two directions, toward stationary mirror 85 and movable mirror 262. Furthermore, the laser light reflected back from the stationary mirror 85 and the laser light reflected back from the movable mirror 262 return to the beam splitter 70 and are combined in the beam splitter 70 and sent along an optical path toward laser light detection unit 34. Here as well, since the movable mirror 262 moves back and forth in reciprocating fashion in the input light axis direction M, the difference in optical path length of the two divided beams changes periodically, and the light which heads from the beam splitter 70 to the laser light detection unit 34 becomes laser interference light whereof the amplitude changes over time. Furthermore, the laser interference light is introduced into laser light detection unit 34. The detection signal, i.e. laser light interference stripe signal produced by this laser light detector is used to compute the location of the movable mirror 262, the movable mirror velocity Vc, etc.
Computer 250 comprises a CPU (control unit) 251 and memory 252, and is connected to a display device 53 and input device 54. To describe the functions processed by the CPU 251 in terms of blocks, the CPU has a light intensity information acquisition unit 251a which acquires light intensity information from infrared detector 21; a movable mirror velocity information acquisition unit 251b which acquires movable mirror velocity information (movable mirror velocity Vc, etc.) from laser light detection unit 34; a movable mirror control unit 251c which controls the movable mirror velocity Vc in the movable mirror unit 260; and a sample measurement unit 251d which computes the absorption spectrum of the sample S.
In cases where a DLATGS detector is used as the infrared detector 21, the DLATGS detector 21 has a frequency characteristic. Thus, if the movable mirror velocity Vc of the movable mirror 262 is not constant, the frequency of flickering of the interferogram becomes non-constant, which appears as measurement error in the absorption spectrum of the sample S. Specifically, if the movable mirror velocity Vc of the movable mirror 262 at the time of background measurement and the movable mirror velocity Vc of the movable mirror 262 at the time of measurement of the sample S differ, there will be baseline distortion of the absorption spectrum and worsening of S/N. Furthermore, during background measurement and measurement of the sample S, accumulation of the IFG signal is performed as the movable mirror 262 repeats its reciprocating movement, and if the movable mirror velocity Vc of the movable mirror 262 changes during accumulation of the IFG signal, there will be a worsening of S/N.
Thus, to make the movable mirror velocity Vc of the movable mirror 262 constant, the movable mirror control unit 251c determines the velocity error (100×(Vc−Vo)/Vo) between the current movable mirror velocity Vc and the target movable mirror velocity Vo and performs feedback control of the voltage applied to the movable mirror unit 260 (the movable mirror application voltage). As a result, the movable mirror velocity Vc is adjusted to the target movable mirror velocity Vo (constant). It will be noted that the “target movable mirror velocity Vo” is stored in memory 252 by the measurer using input device 54. FIG. 9 is a drawing which shows a velocity error signal illustrating an example of the relationship between velocity error and movable mirror position.